Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, middle distillates, naphtha, and gasoline, among others, by hydroprocessing a hydrocarbonaceous feed stock derived from crude oil or heavy fractions thereof. Hydroprocessing can include, for example, hydrocracking, hydrotreating, hydrodesulphurization and the like. Feed stocks subjected to hydroprocessing may include atmospheric gas oils, vacuum gas oils, heavy gas oils, and other hydrocarbon streams recovered from crude oil by distillation. For example, a typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.) and usually at least about 50 percent by weight boiling above 371° C. (700° F.), and a typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydroprocessing uses a hydrogen-containing gas with suitable catalyst(s) for a particular application. In many instances, hydroprocessing is accomplished by contacting the selected feed stock in a reaction vessel or zone with the suitable catalyst under conditions of elevated temperature and pressure in the presence of hydrogen as a separate phase in a three-phase system (i.e., hydrogen gas, a liquid hydrocarbon stream, and a solid catalyst). Such hydroprocessing systems are commonly undertaken in a trickle-bed reactor where the continuous phase throughout the reactor is gaseous.
In such trickle-bed reactors, a substantial excess of the hydrogen gas is present in the reactor to form the continuous gaseous phase. In many instances, a typical trickle-bed hydrocracking reactor requires up to about 10,000 SCF/B of hydrogen at pressures up to 17.3 MPa (2500 psig) to effect the desired reactions. In these systems, because the continuous phase throughout the reactor is the gas-phase, large amounts of excess hydrogen gas are generally required to maintain this continuous phase throughout the reactor vessel. However, supplying such large supplies of gaseous hydrogen at the operating conditions needed for hydroprocessing adds complexity, and capital and operating expense to the hydroprocessing system.
In order to supply and maintain the needed amounts of hydrogen in a continuous gas-phase system, the effluent from the trickle-bed reactor is commonly subject to separation into a gaseous component containing hydrogen and a liquid component. The gaseous component is directed to a compressor and then recycled back to the reactor inlet to assist in supplying the large amounts of hydrogen gas needed to maintain the reactors continuous gaseous phase. Conventional trickle-bed hydrocracking units typically operate up to about 17.3 MPa (2500 psig) and, therefore, require the use of a high-pressure recycle gas compressor in order to provide the recycled hydrogen at necessary elevated pressures. Often such hydrogen recycle can be up to about 10,000 SCF/B, and processing such quantities of hydrogen through a high-pressure compressor adds complexity, increased capital costs, and increased operating costs to the hydrocracking unit. In general, the recycle gas system may represent as much as about 15 to about 30 percent of the cost of a hydroprocessing unit.
In order to eliminate the costly recycle gas compressor, it is has been proposed to use a two-phase hydroprocessing system (i.e., a liquid hydrocarbon stream and solid catalyst) where the continuous phase throughout the reactor is liquid rather than gas. These two-phase systems generally only use enough hydrogen dissolved in the liquid-phase to saturate the liquid in the reactor so a recycle of hydrogen gas is not required, which avoids the use of the recycle gas compressor. However, to ensure that sufficient amounts of hydrogen are dissolved in the liquid phase relative to the unconverted oil to effect the desired reactions, a hydrogen containing diluent liquid is often introduced with the feed so that the ratio of dissolved hydrogen to unconverted oil is high enough to complete the desired reactions at an acceptable rate.
While two-phase systems can operate without a costly recycle gas compressor, the reactions in such two-phase systems are generally less efficient with less contact between the unconverted oil and the catalyst than similar reactions in the more common three-phase systems. For example, with a given amount of catalyst, the contact time of the unconverted oil in the feed with the catalyst in the three-phase system is substantially greater than the contact time of the unconverted oil with catalyst in the liquid-phase system. Generally due to the diluents in the feed of the liquid-phase systems, the contact time of the unconverted oil with the catalyst is reduced substantially because so much of the feed is diluent. As a result, the reaction rates in the liquid-phase systems are less efficient and reduced from those in a three-phase system with a similar amount of catalyst.